With composites the problem is reversed: the material attenuates the ultrasound strongly and the backwall echo comes back weak. High transmission energy is needed, and feasibility has to be verified on the actual laminate.
- ✓ High penetration for fibre laminates and plastics
- ✓ Low frequency and high transmission energy
- ✓ A-Scan indispensable: the echo has to be interpreted, not merely read
- ✓ Measurement on the single layer in multi-material panels
- ✓ Sound velocity derived on the laminate, not taken from a table
- ✓ Feasibility verified on a sample before supply
A metal is homogeneous and returns a clean backwall echo. A composite does not: it is made of fibres and matrix with different acoustic properties, and every interface scatters and attenuates the energy. The result is that the pulse reaches the back wall weakened and returns weaker still; with the transmission energy of an ordinary instrument, often no usable signal comes back at all.
The answer is the opposite of the one used for precision on metals: the frequency is lowered, because low frequencies penetrate further, and the transmission energy is increased. This is the logic on which the PMX4-DL is built, the model in the range dedicated to attenuating materials. What is gained in the ability to get through the material is lost in resolution.
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On sandwich structures the problem grows further: between two composite skins there is a honeycomb or a foam, that is an interface which reflects nearly all the energy. In practice the skin is measured, not the panel: contact ultrasound does not cross the core. This has to be said plainly, because it is a recurring request.
What can actually be measured
| Material | Thickness measurement | Note |
|---|---|---|
| Fibre laminate | Possible with high penetration | Velocity to be derived on the specific laminate |
| Plastics and rubber | Possible with high penetration | Highly attenuating material: check on a sample |
| Skin of a sandwich | Possible | The skin is measured, not the whole panel |
| Honeycomb core | Not with contact ultrasound | The interface reflects nearly all the energy |
Why the test on a sample is mandatory
Two laminates with the same commercial name can behave very differently under ultrasound.
- Attenuation depends on fibre, matrix, volume fraction and porosity
- The sound velocity changes with the lay-up sequence and the direction
- There are no reliable table values: they are derived on the part
- A sample of known thickness of the same laminate is needed
For this reason, before confirming a supply, we verify feasibility on your material: it is the only way to give a serious answer.
Recommended instruments
| Instrument | Why in this application |
|---|---|
| Dakota PMX4-DL | The dedicated model: high energy for composites, plastics and rubber |
| Dakota PMX2-DL | A-Scan and B-Scan to interpret difficult echoes |
| Dakota FX81-DL | Searching for delaminations in the laboratory, with a large display |
| Low-frequency probes | Penetration is obtained by going down in frequency |
Frequently asked questions
With contact ultrasound, no. The honeycomb or foam core forms an interface that reflects nearly all the energy, so the beam does not reach the opposite skin. What is measured is the thickness of the skin the probe rests on, not the whole stack.
Because in composites the sound velocity and the attenuation depend on fibre, matrix, volume fraction, porosity and lay-up sequence: there are no reliable table values as there are for steels. The velocity has to be derived by measuring a sample of known thickness of the same laminate.
A thickness gauge measures thickness, it does not look for flaws: a delamination can make a reading appear anomalous, but interpreting it requires the A-Scan and is not equivalent to a flaw detection inspection. For the systematic search for delaminations a flaw detector with a dedicated procedure is used.
Back to Aerospace and Defence. For a feasibility check on your laminate, write to us stating the type of fibre, the matrix and the nominal thickness.
Further information and contacts
Systems division and sensor integration partner : www.roder.it
More information about RODER SRL : about us
Contact for general information : info@roder.it
Contact for information about these products : info@roder.it
The information contained on this web page is provided for information purposes only. Although it has been compiled with the utmost care, it does not constitute a contractual offer or a binding commitment to supply. It may contain transcription, translation or typographical errors. For precise and up-to-date information, please contact our company directly.
Some images on this website have been intentionally generated using Artificial Intelligence (AI). This is because, for many applications and projects, it is not possible to disclose photographs of the installation or of the actual system due to confidentiality agreements, confidentiality clauses and non-disclosure agreements (NDAs).